The present invention relates to a method for preparing diaryl iodonium salts.
Diaryl iodonium salts containing as a pair ion a halogenated metal represented by the general formula EQU [Ar--I.sup..sym. --Ar']MX.sub.n.sup..crclbar.
wherein Ar and Ar' are an aryl group, respectively, X is a halogen and n is an integer of 1 or more have drawn an attention in the art as an initiator for the photo polymerization of cation polymerizable monomers. Such compounds are prepared from a diaryl iodonium halide or a diaryl iodonium bisulfate.
Methods for preparing the halide heretofore proposed are such methods as
(A) coupling of an aromatic hydrocarbon with an aryl iodide by the use of an oxidizing agent such as a persulfate; PA1 (B) the coupling of an aromatic hydrocarbon by the use of a periodate, iodyl sulfate [(IO).sub.2 SO.sub.4 ] or iodine acrylate [I(OCOR).sub.3 ]; PA1 (C) coupling of the oxidized product of an aryl iodide with an aromatic hydrocarbon; and PA1 (D) the method by the use of an aryl lithium.
The method (A) above is described by Beringer et al. in J. Am. Chem. Soc. 81, 342 (1959). According to the method of Beringer et al., a coupling reaction of the iodine-substituted product such as nitrobenzene or benzoic acid with benzene is carried out in the presence of concentrated sulfuric acid and a persulfate, and an unsymmetric diaryl iodonium salt is recovered.
It was demonstrated by us, however, that when concentrated sulfuric acid was employed as in the above-mentioned method of Beringer et al. for a coupling reaction of an aromatic hydrocarbon substited with allyl or alkoxy group with an iodine-substituted derivative thereof, such reactions as sulfonation predominate to lower the yield of the diaryl iodonium salt. Also, the purity of diaryl iodonium salt thus obtained is very low and the purification is quite difficult. We have unexpectedly found that the reaction in a sulfuric acid solution diluted to a predetermined concentration is not accomplished by such reactions as sulfonation thereby improving the selectivity and giving a diaryl iodonium salt in a higher yield.